The National Labor Relations Board has ruled it is illegal to force union-backing employees to attend meetings held by their companies that are designed to undermine support and argue against the labor groups.

The decision passed 3-1, with the NLRB General Counsel, Jennifer Abruzzo, arguing the meetings violated worker’s rights, as they were forced to attend and be subjected to the company’s anti-union messaging.

Mandatory “captive audience” meetings in which companies argue against unionization are illegal, the National Labor Relations Board ruled in a case involving Amazon.com Inc., prohibiting one of employers’ most potent weapons against labor organizing campaigns.